I picked up an HP 400FL AC voltmeter in good condition except that the meter movement won't zero. I don't see any zero adjust; the needle is resting to the left of zero with a fair amount of tension on it. I pulled the movement and there are two assemblies on top and bottom that look similar (see photos), but they are glued into position. I can't see that anything has come loose. It is possible this meter has been dropped, although there is no sign of damage.Is this movement a goner?

What you have there is a Taut-Band suspension meter. Rotating one of those two assemblies will zero correct the meter. They are sometimes glued in place by the factory to keep the zero set point from drifting, when there is no external corrector screw to adjust it. This particular movement looks like it would probably be corrected primarily by the rear adjuster.

I am however unfamiliar with that particular assembly, though it appears to be a very high quality suspension, with multiple adjustment points.. More pics would help. Especially of the rear.

From what I understand, the HP 400FL did not allow for meter zeroing. If you Google the images of this unit, you will see that a fair number of the meter faces shown on apparently operating units all have the needle resting below zero. Of particular interest might be the posting on the Geller Labs page:

It contains a link to a .pdf copy of the manual which states that only the 400F has the mechanical meter zero apparatus. These are magnificent beasts and can be put right (calibrated and repaired) if one pays strict attention to the manual.

Of course, the 400 FL differs from the 400 F unit in that it has the log/db scale whereas the 400 F unit has a predominantly voltage scale. Note that a sister instrument, the HP 3400A unit, has a sort of "no-man's" land (no tick marks) between the "1" and "0" on the main scale. It was also prone to a start-up "meter-mambo" during warm-up. Of interest might be section 3-9 in the manual:

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Manual Sec 3-9.jpg [ 56.51 KiB | Viewed 2466 times ]

Good luck.

_________________Bill, k6whp"Cheer up, things could get worse.So I cheered up and things got worse."

Am I to assume that the voltmeter, when turned on, should have a pointer that rests on 0 (?)

If so, then the 400FL still has problems, but not with the meter movement. When I first got it, the pointer would slam to the right, then left, when changing ranges. I replaced a couple of capacitors and it is working better but not perfect; it is possible someone has been inside with their golden screwdriver. My next step will be to get the manual.

Am I to assume that the voltmeter, when turned on, should have a pointer that rests on 0 (?)

If so, then the 400FL still has problems, but not with the meter movement. When I first got it, the pointer would slam to the right, then left, when changing ranges. I replaced a couple of capacitors and it is working better but not perfect; it is possible someone has been inside with their golden screwdriver. My next step will be to get the manual.

Thanks for the help so far.

In theory, if the meter were noiseless and the input shorted, the needle should stay against the stop. Given that it actually has noise, it will read upscale. The manual should say how much to expect.

My HP 400GL is the gray paint version about 2 years older than yours but the same schematic and meter movement. I had it on my bench at work from 1973 and when the company shut down I was allowed to take it home. The meter always reads below scale with power off or with power on and no signal. When you power it up it pegs full scale then returns down scale. Did that from day one. It has stayed in cal with no adjustment since it was new. Don't worry yours sounds normal. The movement is set so that it takes about 25% of full scale torque to read the first value on the scale to improve linearity. These are great meters.

If you look at the schematic you will see that on power up the meter drive gets unbalanced because of the way the power supplies come up. Since the meter amp limits the drive current that does not hurt the movement.

Some of the caps are teflon which are far superior to modern film caps so I would not get too far into parts subbing. Also some of the electrolytics are far better than ordinary ones today for esr at high frequency so unless they get leaky which is really rare swapping them out for modern caps will raise the noise floor at HF. Mine still has the original parts and they still work fine.

The meter should read below scale on all ranges with a 1K resistor across the input.

This meter has good response out past 10Mhz so it also uses wet slug tantalum caps in many places to bypass noise and keep the open loop gain up on the amplifiers. they may look like electrolytics with a + marking but subbing any electrolytic in those locations will screw up the HF response and noise floor badly. Any of the smaller value caps do not have modern replacements and should never fail in the next 50 years. Modern dry tantalums are not a great sub as HF impedance and noise are far worse.

If this meter has been sitting for years like some have it can take several day of operation to restore the tantalums and electrolytics to the leakage values they need.

The best thing for this meter is to leave it on for a week or so before trying to fix anything except smoke.

The input attenuator switch is sliver plated and may need a few cycles to clean up but contact cleaner can cause problems with the high input impedance.

Realize this is an old thread but I wanted to drop in and note that the HP400FL is an HP400F with a "log," linear dB, top scale meter movement. That's the "L" in the model number.

I bought one of these fine meters - the L version - back in the late 70s. I now own a 400F and a 400FL.

The HP 400FL does not have an internal log converter: The linearization of the dB scale is done mechanically internal to the meter. This may be unique to HP.

It is normal for the meter to rest on the left hand peg if it's below scale since -10 dB is what is normally 0 volts on the linear scale version.

The wet slug tantalum caps do need a significant amount of time to re-form if the unit has been unpowered. As mentioned earlier I do not reccomend replacing them unless absolutely necessary. They are very low leakeage (when formed) and do have low ESR/ESL. Replacing them with conventional aluminum electrolytics could cause response anomalies.

Dry tantalum caps short. Wet tantalum caps typically corrode the (+) lead, or leak from that end. YMMV. I saw tons of them in Navy avionics gear, and tons more in Tek and HP equipment. Engineers of old didn't leave much overhead for voltage rating, with 35V being the typical rating for a +28V bus.

I've got tons of salvaged-but-good mil-spec stuff, mostly dry.

Save those bad tantalum caps - and especially the silver cased ones:http://recycletantalumcapacitors.com/ <-- I did sell Eric some $140 of NOS tantalums I got at auction. Mostly 70s date codes, and more than I could ever use. Good guy, a regular over on the Gold Refining Forum. Pays out via Paypal.

The HP 400FL does not have an internal log converter: The linearization of the dB scale is done mechanically internal to the meter. This may be unique to HP.

The General Radio 1806-A Electronic Voltmeter uses a meter with a (mostly) logarithmic movement. So HP was not unique. The Ballantine meters with logarithmic scales, however, do use an electronic log converter.

Edit: I was wrong. Ballantine instruments did use meters with a logarithmic movement. The original 300 used a logarithmic meter according to Rider's VTVM book. I also noticed the catalogs from the 60's list many meters such as the 300E and the 300G with "shaped pole pieces to achieve a logarithmic characteristic over a 10 to 1 voltage range."http://lcweb2.loc.gov/master/mbrs/recor ... 20(catalog).pdf

Ballantine may have produced some VTVMs using electronic log converters based on variable-mu tubes, as Rider cites a few articles written by Stewart Ballantine on log converters.